Question 1

What is useEffect for?

Accepted Answer

Synchronize with external systems after render: subscriptions, timers, non-React widgets. Not for computing derived state. Specify dependencies. Return a cleanup function.

Question 2

Why does an effect run twice in React Strict Mode (dev)??

Accepted Answer

React mounts, unmounts, remounts to surface missing cleanup bugs. Write resilient effects with proper cleanup. Don't disable Strict Mode to 'fix' double fetch without understanding.

Question 3

How do you correctly depend on props/state in effects?

Accepted Answer

List every reactive value used; use functional updates; extract stable callbacks or use refs for latest values intentionally. Exhaustive-deps warnings matter. Avoid empty deps when you read changing values.

Question 4

Fetching in useEffect — modern guidance?

Accepted Answer

Works but race-prone; prefer frameworks/data libraries (React Query) or server components where appropriate. AbortController on cleanup. Don't ignore race conditions when query changes quickly.

Question 5

When is useLayoutEffect preferred?

Accepted Answer

When you must measure/mutate DOM before paint to avoid flicker. Blocks paint—use sparingly. SSR warnings: layout effects don't run on server.

Question 6

Effect cleanup examples?

Accepted Answer

Clear timers, abort fetches, unsubscribe events, disconnect observers. Missing cleanup causes leaks and duplicate handlers. Always pair subscriptions with teardown.

Question 7

Anti-pattern: chaining many effects to mirror state. What instead?

Accepted Answer

Derive during render, use events to update state directly, or reducers for transitions. Effect chains are hard to reason about and cause cascading renders.

useEffect React Interview Questions

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